First of all, "fluent" generally refers to speaking and being able to express oneself comfortably, so "fluent in reading" or "fluent in manga" doesn't really make sense as a concept. ("literate in Japanese", or "able to understand manga comfortably" would make more sense and be easier to answer.)
Secondly, Japanese courses vary in intensiveness and in quality, and learners studying in such courses vary in their effort and ability.
There is no such thing as one course that will make any learner "fluent" or "literate" in Japanese. It is about the quality of the course and the effort of the learner. If you put in the time and effort, you can achieve anything. If you don't, you can achieve nothing. I have seen both cases over the years. It's not an attack on anyone, but both are possible.